This invention relates to a gun shot control unit of the type used to control the firing of seismic sources and to a method of seismic exploration using this gun shot control unit with an on-bottom seismometer.
In seismic exploration, it is frequently desirable to fire the seismic source at precise times. For example, copending application, Ser. No. 163,757, filed June 27, 1980, Bowden, et al discloses an on-bottom seismometer unit in which a tape recorder is turned on and off at precise times to record seismic records produced from a source which is fired at precise times. Before the on-bottom seismometer is deployed, its clock is synchronized with the clock which controls the seismic source. Because of this, the record times in the seismometer are synchronized with the times at which the seismic source is fired. However, almost all seismic sources have an inherent delay between the time that they are commanded to fire and the actual occurrence of the seismic pulse. This inherent delay includes the mechanical lag of the mechanical parts which make up a typical air gun, for example. This inherent delay varies from source to source and may vary from time to time.
The prior art has recognized this inherent delay and has provided means for minimizing its significance. The principal thrust of prior art attempts to compensate for gun delays has been to synchronize the production of pulses from multiple seismic guns. It is desirable to precisely synchronize the production of seismic pulses from these many guns. It may be desirable to fire the guns simultaneously, or in a pattern with precise delays between the the seismic pulses from the many guns. In either event, the prior art has detected the times of firings of the guns, determined the time differences between the gun firings, and provided means for adjusting the times of firing of the various guns so that the desired firing pattern is obtained. For example, in U.S. Pat. No. 3,686,218 Ritter, the seismic sources are fired in response to a signal FC.sub.o from the recording system. The firing command delay unit 12 introduces variable delays which are settable by the switches 10. At a variable time, programmed by the switches 10, after the system firing command FC.sub.o, the firing commands FC.sub.1, FC.sub.2, FC.sub.3, and FC.sub.4, are generated. These fire the guns. The signals TB.sub.1, TB.sub.2, TB.sub.3, and TB.sub.4 represent the actual time of firing of the guns. The variations between the system firing command FC.sub.o and the actual firings TB.sub.1, TB.sub.2, TB.sub.3 and TB.sub.4 are displayed in the display unit 16. If these variations are different from the desired delay, 20 milliseconds in the example under discussion, the switches 10 are changed to adjust the delay to that desired. Such a system is very useful in marine seismic exploration. However, when a seismic source is to be used with an on-bottom seismomenter of the type under consideration, one further capability must be provided. The gun control system must have the capability of precisely synchronizing the firing of the guns at precise times which coincide with times derived from a clock in the on-bottom seismometer. While the Ritter system has the capability of firing the guns at a variable time after the occurrence of a synchronizing pulse, i.e., FC.sub.o, it does not have the capability of firing in synchronism with this signal. Also, the Input Output Company (IO) in Houston, Tex. supplies a gun controller which fires four guns. This system has a digital display for displaying the relative time of firing of all guns. From this digital display the operator can adjust the firing time, so that they are all equal, or in the desired timing pattern.
Systems of the type just described are desirable in all types of seismic exploration, including the on-bottom seismometer application referred to above. However, such a system does not provide the capability of synchronizing the firing of the the guns at precise times which coincide with times from a clock in a remote recording system.